Floating roof

ABSTRACT

A floating roof suitable for use with large capacity storage tanks comprises a central framework surrounded by one or more concentric floating rims. The rims are connected to the framework by tensioning cables that are adjusted to give a stable structure. The roof also comprises a covering sheet that is supported by the framework and the rims. A further embodiment of the roof has one or more of the rims located within the area of the central framework giving a more flattened roof suitable for some types of smaller capacity storage tanks.

Umted States Patent 11 1 1111 3,85,699

Pladys May 27, 1975 [54] FLOATING ROOF 2,282,772 5/1942 Wiggins 220/26 R2,282,773 5/1942 Wiggins 220/26 S [75] Inventor: Nest" Pladys,Rosendael1 2,586,856 2/1952 Orr et a1. 220/26 R France 2,606,684 8/1952Wiggins 220/26 R 2,968,420 1/1961 Harris et al. 220/26 SA [73] Assgnee'{:Egf'ij ggg s gg g 3,116,850 1/1964 Anderson 61 al. 220/26 SA [22]Filed: Sept 1972 Primary Examiner-William 1. Price [21] APPL 290,761Assistant Examiner-Stephen Marcus Attorney, Agent, or Firm-Morgan,Finnegan, Durham & Pine Foreign Application Priority Data Sept. 20, 1971France 71.33779 ABSTRACT [52] US. Cl. 220/225; 220/216; 220/219 Afloating roof suitable for use with large capacity [51] Int. Cl Bd 87/20storage tanks comprises a central framework sur- [58] Field of Search220/26 R, 26 S, 26 SA, rounded by one or more concentric floating rims.The

220/26 D, 216, 219, 220, 225 rims are connected to the framework bytensioning cables that are adjusted to give a stable structure. The [56]References Cited roof also comprises a covering sheet that is supportedUNITED STATES PATENTS by the framework and the rims. A furtherembodiment l 121 057 12/1914 Wilcox et al 22006 R of the roof has one ormore of the rims located within 1:267 434 5/1918 MacPherson:::::::::::i:::i:iii 220 26 R the area Of'the Central framework giving amore 1,5202991 12 1924 Wiggins 220/26 R tened roof Suitable for some ypof Smaller Capacity 1,994,478 3/1935 Kueffer 220/26 R Storage tanks.2,072,798 3/1937 Cranz 2,281,748 5/1942 Carney 220/26 s 24 Clalms, 12Drawmg Flgures mac SHEET Q% I [III Q lili Ill SHEET FLOATING ROOF Thepresent invention relates to a floating roof suitable for use with aliquid storage tank and being particularly suited for use with storagetanks of large dimensions, such as those intended for the storage ofliquid petroleum products.

It is desirable to cover a volatile liquid such as gasoline, crude oiland the like, which is contained within a storage tank, with a roof ordeck which normally floats on the surface of the liquid and provides avirtually vapour tight shield to minimise loss of liquid by evaporation.

These storage tanks now constitute an increasing part of the investmentin the petroleum industry and the growing size of the tanks giveproblems in the construction of floating roofs of large diameter. It iswell known for a floating roof to comprise a large diameter circulardeck joined at the edge to an annular pontoon. These large decks sufferthe disadvantage of being heavy and are particularly vulnerable todeformation under the action of external stresses e.g., gusts of wind,weight of snow, and to movement of the liquid stored.

According to the invention there is provided a floating roof suitablefor use with large capacity storage tanks which comprises a centralframework with one or more substantially concentric floating rims, therims being connected to said framework by means of tenstoning cablesadapted to give a stable structure and a covering sheet adapted to besupported by the framework and rims.

The central framework preferably comprises vertical floats braced attheir upper and lower parts by crowns to which are attached totensioning cables adapted to ensure the concentricity of the floatingrims.

Preferably each tensioning cable passes over a pulley adapted to fit onthe rim.

The roof proper preferably Consists of an assembly of thin sheetsstiffened by parallel ribs and which are fixed on to the above-mentionedfloating rims so as to have a slight slope towards the centre or towardsthe outside, thus ensuring that any rainwater flows towards a Centralwell or annular well provided for this purpose- For the purpose ofproviding leakproofness between the said roof and the peripheral wall ofthe tank, the roof may be provided with a seal comprising an articulatedcollar to the end of which there is fixed a circular attachment whichcan be inflated with water or by means of an inert gas or compressedair, this attachment being coupled to another attachment of the sametype which plays the part of a restoring spring and ensures theapplication of the first joint against the peripheral wall of the saidtank, the assembly of these two omts supported continuously by th id tiulated collar operating in accordance with the principle of leyersbetween the peripheral part of the external floatmg rim provided in thevicinity of the periphery of the said roof and the said peripheral wall.i

The effects of the wind are attenuated by means of brackets possessingsuitable respective heights and fitted with horizontal stabilisingplates of a suitable area adapted so as to rest on the bottom of thetank when the latter is empty. These brackets act as stabiliscrs or asacks in the stored liquid by damping the movements of the roof due toexternal stresses because of the incrtin of the oil with which thestabilisers are in contact.

The ill'l()\ki-l'llClTll(lllC(l framework may be made of steel, but italso may be made of any other rigid material, and particularly of asynthetic plastic material.

The central framework comprises standard commercial tubes which mayhave, for example, a thickness of 12 to ZO/ll) of a millimetre. Thesetubes may possibly be of the clipped spiral type.

However, in order to avoid costly operations of shaping, these rims arepreferably given a polygonal crosssection by making them of an assemblyof linear tubular elements, always cut at the same angle at their twoends, for example at an angle of 50, these tubular elements then beingassembled by standard sleeves on which the cable attachments are welded.

Preferablythe tensioning cables are encased in a suitable plasticsmaterial such as polyvinyl chloride for example. In order to render thefloats unsinkable, it is advantageous to fill them with a synthetic foamproduct, such as for example, polyurethane foam.

All the metal elements may be treated to render them corrosion resistantagainst the effects of the petroleum products stored and atmosphericagents.

The covering sheet, preferably, comprises panels of shaped material thatare juxtaposed and interconnected. The panels are connected togethereither directly or by means of junction pieces of suitable shape alongthe edges of said panels. Most preferably the panels have corrugationsat right angles to the elements of the rim,

In order to ensure the rigidity of the roof despite its thinness, it ispossible to use pre-fabricated sections assembled together byleakproofjoints arranged parallel to the said ribs so as to avoidwelding if possible. The foils arranged between the said joints will bereferred to below as panels and may possibly co-operate with parts of atriangular shape located between two neighbouring floating rims.

The above-mentioned panels, if they are used, may also be renderedcorrosion resistant.

The tension exerted by the said cross-cables shows itself as acompression uniformly distributed in the above-mentioned floating rimsand in the vertical floats of the hub and by a flexural moment in thecoupling sleeves of the straight elements of the floating rims, thismoment being balanced by a reinforcement of the said sleeves,particularly by a horizontal rib.

In the event of tensions which are too large being exerted on certain ofthe cables which are taut, the risks of collapse are reduced by the factthat the sheets constituting the roof form braces which make it possibleto limit the said compressive forces.

In order to avoid excessive loading by snow, the roof may be equipped onits outer surface with an electrical heating device.

Preferably the mean thickness of the elements making up the new roof,including that of the floats, the rims and the cables, does not exceed 4mm. For purposes of comparison this thickness is of the order of 7 mm.for floating roofs of the single deck type and of the order of i3 to 14mm. for floating roofs of the double deck" type. The relative thinnessof the roof may yield a consequent weight reduction compared tconventional floating roofs.

The tensioning cables for the roof constitute a Faraday cage thusprotecting the roof against lightning and helps to reduce theaccumulation of static electricity.

Although the roofs according to the present inven tion are of maximumadvantage when used with storage tanks of a very large diameter. (forwhich conventional floating roofs are not always suitable). the new roofaccording to the invention may also be adapted to tanks of a lowcapacity of most types, and particularly to those currently used whenthe bottom of the tank is a horizontal plate.

Thus according to a further aspect of the invention there is provided afloating roofin which one or more of the floating rims lie within thearea of the central framework.

In this case the shape of the roof is more flattened so as to avoid aloss of storage when the roof is in the low position. It is in factimportant to prevent the tank from breathing for obvious reasons ofsafety. The level of the petroleum stored, when the brackets rest on thebottom, must therefore be minimum. The roof is flattened whilst ensuringits mechanical strength by removing the hub from the centre and crossingthe end tightening cables around the floating rims.

In a further embodiment of the floating roof, this roof comprises inaddition an integral leak proof structure, the framework being of thesame type as in the first embodiment of the invention but the sealingfoil is in this case double and consisting of a first sheet connected tothe above-mentioned upper tightening cables and resting on these cablesso as to constitute a tent inclined from the centre towards theperiphery and a second sheet intended to rest always on the surface ofthe stored liquid between a peripheral rim and the median part of thecentral hub.

Thus according to a still further aspect of the invention there isprovided a floating roof which comprises only one floating rim, said rimbeing connected to the central framework by tensioning cables attachedto an upper and a lower part of said framework and a covering sheetformed in two parts, one part of which is supported by the upper sectionof said tensioning cable and the other part being attached to thecentral framework and floating rim, the two parts of said sheet beingadapted to combine with the rim to form a leakproof enclosure. Theassembly thus formed constitutes a leakproof structure which can bepressurized (to about 0.01 0.02 bars) to reduce the effect of the weightof the upper sheet on the cable and to allow the lower sheet to rest onthe surface of the stored liquid.

Such a pressurised embodiment furthermore prevents any furtherpossibility of the introduction of petroleum vapour or explosive vapourinto the pressurised enclosure.

Furthermore, in this embodiment, the slope of the upper sheet ensuresthe flow of rainwater towards the periphery of the tank.

It is also possible to provide a device for heating the air in theabove-mentioned pressurised enclosure so as to prevent. for example, anaccumulation of snow in winter on the upper sheet.

In the event of the accidental lack of pressurisation, the consequencesare limited since the two sheets are both supported.

Furthermore, in this embodiment, the hub only has one central stabiliserwhich can possibly slide in a vertical tube planted in the centre of atank, it being possible for this vertical tube to be stayed so as toensure that it is kept vertical.

The invention will now be described by example with reference to thedrawings accompanying the Specification. The drawings show three typesof floating roofs suitable for use with storage tanks according to theinvention in which:

FIG. I is a cross-section in elevation of storage tank equipped with afloating roof according to the inven-.

tion.

FIG. 2 is a cross-section in elevation ofa floating roof of asemi-buried storage tank.

FIG. 3 represents an alternative embodiment of the invention in the formof a flattened floating roof in a virtually empty tank having its hubfurther away from the centre and located around a floating rim.

FIG. 4 is a section along line IV IV of FIG. 5.

FIG. 5 is a plan view of the floating roof of FIG. 2 showing thearrangement of the crossed tensioning cables of the floating rims whichare concentric in relation to the central hub and also showing a part ofthe sheet of the said roof.

FIG. 6 is a complementary diagrammatic view of the same floating roof asFIG. 5 showing the tensioning cables between one rim and the saidcentral hub.

FIG. 7 is an enlarged section along line VII VII of FIG. 5 through theconnecting device between the tensioning cables and the assemblingsleeves of the tube elements constituting the floating rims.

FIG. 8 is a plan view of part VIII of FIG. 5.

FIG. 9 is a cross-section in elevation of a storage tank similar to thatof FIG. I equipped with another embodiment of a floating roof having adouble sealing sheet.

FIG. 10 shows a cross section in elevation of a semiburied storage tankalso equipped with a floating roof having a double sealing sheet.

FIG. 11 is a plan view of the roof corresponding to FIGS. 9 and 10.

FIG. 12 is an enlarged cross-section in elevation of the roofcorresponding to FIGS. 9 and 10.

A typical use of the floating roof is with storage tanks containingliquid petroleum products.

FIGS. 1 and 2 show the central hub l of the framework of the floatingroof and the crowns 2, 3, 4 and 5 on which there are moored thetensioning cables of the different concentric floating rims 6 to 10.These rims are arranged, according to a polygonal contour whichfacilitate the assembly of the consecutive linear units of these rims.

They are equipped with brackets 6a to 10a of appropriate length, in sucha way that the stabilisers 6b and 10b can come to rest on the peripheralpart 11 of the bottom part of the tank on which the said floating roofis to be used.

This tank has a circular peripheral wall 12 which is located in theimmediate vicinity of the outer rim I0, which is preferably of polygonalshape.

A device 13 for pumping the stored liquid is mounted on the saidfloating roof by means of articulations which can be seen at I4 of FIGS.1 and 5, and which permit this device to follow the variations of thelevel of the free surface of the stored liquid. The device. may besubstituted with suitable equivalent existing devices.

In FIG. 2. in the centre of the tank there is a central well 15 at thebottom of which it is possible for the rainwater to collect, this waterarriving through a watertight pipe I6 passing through the stored liquidafter first of all following the slope of an upper sheet constitutingthe roof proper 17, which is inclined towards the centre in thedirection of the free surface of the stored liquid. The collection ofrainwater is not represented in FIG. 1.

For purposes of simplification, the crossed cables which serve to ensurethe concentricity of the rims 6 to have been omitted in FIGS. 1 and 2.

These crossed cables, which are shown in FIGS. 5 are arranged as in FIG.6. When they are above the horizontal median plane they are shown insolid lines and in broken lines when they are below this plane.

The hub and the rim under consideration are only shown by circlescorresponding to the points of attachment of the acid cables. A cablebearing the reference no. 18a is anchored to the lower crown of the hubat the point 5a. It passes into the pulley 21a and then, in the upperposition 18b, rejoins the upper crown at the point 2a shown in thedrawing at 60 in relation to the anchorage 5a, and it is at this point2a that the cable 18 (18a plus 18b) is anchored and stretched.

The immediate neighbouring cable 19b is situated at in a clockwisedirection in front of the cable 18a. It is anchored and stretched atpoint 2b of the upper crown of the hub, passes into the pulley 21b,becomes 19a, then is anchored to the point 5b of the lower crown of thehub.

Thus the cables oriented in the same manner on FIG. 6 are alternativelyabove and below the horizontal median plane, and the anchorage pulleyson the floating rims form with the corresponding radial planes angleswhich are turned alternately to the left and to the right.

This symmetrical arrangement renders the system virtually undeformableby forces in both the vertical and radial planes as well as by rotatoryforces whilst imparting to it the desirable elastic properties.

The hub 1 consists of a certain number of vertical floats 1a which canbe seen in FIGS. 1 to 5. The free ends of the cables 18 and 19 forcentering the floating rims 6 to 8 are moored on to crowns 3 and 4 ofthe said central hub, whilst those of cables 18 and 19 for centering thefloating rims 9 and 10 are moored on the outer crowns 2 and 5 of thiscentral hub.

As regards the sheet 17, the slope of which makes it possible to collectthe rainwater in the well 15 through the pipe 16, it can be seen inFIGS. 4 and 5 that this sheet consists of the juxtaposition of sectionsheets of given width known as panels and bearing the references 17a inFIG. 4, each of these panels occupying the space between two parallelsections of FIG. 5, being assembled together along the joints l7b whichcan be seen in particular in FIG. 4.

The panels also have intermediate ribs of greater or lesser amplitudereferred to respectively by the reference numbers 170 and 17d.

The length of the inclined sides of the trapezoidal elements thusconstituted, the junctions between the joints 17b of the outer panelsare made by means of shaped pieces (or fillet sections) 17e which alsoenable assembly by tight fitting on the joints 17b of the neighbouringtrapezoidal units.

The sealing sheet is equipped with accessories such as an access ladder,vent holes, and suction valves to permit protection of the tank andallows breathing when the roof is in its low position. For the sake ofclarity, these accessories have not been shown in the draw- %n FIG. 5 itis also possible to see the stabilisers which are square in shape, 6b to10b, and the different concentric floating rims 6 to 10, consisting oflinear elements connected together by 24 sleeves of appropriate shape20, which can be seen particularly in FIG. 8, and on which there aremoored the cables 18 or 19 via the pulleys 21 mounted on plates 22connected by means of two rings 23 which are particularly visible inFIG. 7, to a plate 24 which is integral with the sleeve 20.

The bracket 10a is extended in its upper part, similarly to the otherbrackets, by means of a tubular unit welded on to the rim 10 which alsocarries between the said elements We longitudinal brackets 10d whichsupport the sheet 17.

Each tensioning cable 18 or 19 consists of two sections, the upper one19a, and a lower one 18b, 1%, the free ends of these two sections beingfixed respectively to the symmetrical crowns 2 and 5 or 3 and 4 locatedsymmetrically in relation to the level of the stored liquid in the tankin question.

The detail of assembly between the elements of the rim 6 to 10 and thesleeves 20 can be seen in FIG. 8 and comprises the use of internalsleeves 25 mounted on the elements 20 and on the outside of which thereare fixed the linear elements making up the rims 6 to 10.

FIG. 7 shows the detail of articulated collar 28 which includes theleakproof joint 26 mounted on the opposite side of collar 28 between theframework of the floating roof and the skirt of the tank. Joint 26 isinflated by means of a liquid, an inert gas or with compressed air, andanother symmetrical joint 27 mounted on the opposite side of collar 28,is similarly inflated to press joint 26 against the cylindrical wall 12of the tank by lever action thus ensuring a seal.

The lower part of collar 28 of the articulated supports of the joint 27is polygonal and this lower part is pivotally mounted at 29 on thesupports 30 fixed along the sides of the polygonal rims 6 to 10 toprovide a pivot.

The upper part 31, which ends in a cylindrical support 31a of the joint26, is of a left-hand shape which ends in a circle in its upper part anda polygon in its lower part 32.

FIGS. 912 show an alternative embodiment of the roof having a doublesealing sheet. The different concentric rims 6 to 10 which are visiblein particular in FIG. 5 are replaced by a single peripheral circular rimintended to be applied against the peripheral wall 12 of the tank whichis to receive the said floating roof.

In this embodiment, the sheet 17 is replaced by a double sheetconsisting of an upper sheet 33 made of plastics material resting on theupper part of the radial cables 34 for tensioning the said rim and of alower sheet 35 which is more or less horizontal, also made of plasticsmaterial and resting on the upper part of the stored liquid, as well ason the central cables 36 also arranged radially.

FIG. 12 shows in greater detail the cables 34, which instead of beingmoored to the inner part of the floating rim, whose concentricity theyhave to provide, pass inside the said floating rim 37 following theouter part of this rim and being fixed by their free ends on to a hub lbof a type very different from hub 1, which is capable of sliding on amast 38 which rests on the centre of a central well 15 (FIG. 10).

The mast 38 may be stayed, for example on the slopes of the retainingwell in the case of semi-buried tanks. This stage is not shown in FIG.10.

The hub lb is fixed to a stabiliser device consisting mainly of ahorizontal plate 1c playing the part of shock absorber and preventingthe oscillations of the hub 1b during the rapid descent of the level ofthe free surface of the petroleum liquid contained in the abovementionedtank.

In this embodiment applied to semi-buried tanks, the lower part of thetightening cables 34 comes into contact with the peripheral part 1 1 ofthe bottom of the tank when the tank is empty.

The two sheets 33 and 35 form a pressurisable enclosure which tends toprevent the upper sheet 33 from weighing on the upper part of thetensioning cables 34 and tending at the same time to apply the sheet 35against the free surface of the stored liquid.

Moreover, this pressurised enclosure is at a pressure which issufficient to prevent any re-entry of petroleum vapour or of anexplosive mixture between the two sheets 33 and 35.

In the enlarged view of FIG. 12 one can see the sheets 33 and 35 and thetwo sections of cable 34, the median part of which passes into a groove40 provided inside the floating rim 37.

In this embodiment, not only the rim 37 comes into contact with theperipheral wall 12 of the tank, but the sheet 33 covers the rim 37 and33a, which ensures the flow of rainwater towards the outer part of thefloating roof. The rim 37 is integral with rods 37a which end instabiliser plates 37b which can also be seen in FIG. 11.

I claim:

1. A floating roof suitable for use with a large capacity storage tankcomprises (i) a central framework and at least one floating rim, (ii)tensioning cables connected from said central framework to said floatingrim, (iii) a covering sheet adapted to be supported by said centralframework and said floating rim, and (iv) a seal comprising anarticulated collar pivotally attached to the outermost floating rim,said articulated collar being fitted with a first inflatable attachmentand a second inflatable attachment which is nearer to the pivot than thefirst inflatable attachment, said attachments being on opposite sides ofsaid articulated collar and being adapted so that upon inflation of saidinflatable attachments, the second inflatable attachment is pressedagainst the rim and by a level action presses the first inflatableattachment against a surrounding wall.

2. A floating roof according to claim 1 wherein there are a plurality ofrims and all the rims are located outside the said framework.

3. A floating roof according to claim 1 in which at least one of saidrims is located within the area of said framework.

4. A floating roof according to claim 1 in which each tensioning cablepasses over a pulley adapted to fit on the rim.

5. A floating roof according to claim 1 in which the covering sheetslopes down towards the central framework.

6. A floating roof according to claim 1 in which the said sheet isfabricated from a plastics material.

7. A floating roof according to claim 1 in which the floating rim issubstantially circular in form.

8. A floating roof according to claim 1 in which the rim issubstantially circular in cross section.

9. A floating roof according to claim 1 in which the tensioning cablesare arranged so that the rotational forces applied to adjacent elementsof the rim are in opposition.

10. A floating roof according to claim 1 in which the covering sheet isadapted to permit collectionand removal of a liquid which may fall onits surface.

11. A floating roof according to claim 1 in which the central frameworkat least one vertical float.

12. A floating roof according to claim 11 in which said centralframework includes a plurality of vertical floats; each upper part ofsaid floats is connected together and each lower part of said floats isconnected together by means of upper and lower crowns, respectively.

13. A floating roof according to claim 12, in which each tensioningcable is attached to the upper and lower crowns of the centralframework.

14. A floating roof according to claim 1 in which the covering sheetcomprises panels of shaped material that are juxtaposed andinterconnected.

15. A floating roof according to claim 14 in which the panels of shapedmaterial are connected together, by means of junction pieces of suitableshape, along the edges of said panels.

16. A floating roof according to claim 14 in which the panels ofmaterial have corrugations substantially at right angles to the elementsof the rim.

17. A floating roof according to claim 1 in which the floating rims arefitted with stabilizers for supporting the roof on the bottom of thetank when the tank is empty.

18. A floating roof according to claim 17 in which the stabiliserscomprise substantially vertical legs fitted with horizontal plates.

19. A floating roof according to claim 1 which comprises one floatingrim, said rim being connected to the central framework by tensioningcables attached to an upper and a lower part of said framework and acovering sheet formed in two parts, one part of which is sup ported bythe upper section of said tensioning cable and the other part beingattached to the central framework and floating rim, the two parts ofsaid sheetbeing adapted to combine with the rim to form a leakproofenclosure.

20. A floating roof according to claim 19 in which the leak-proofenclosure is adapted to be pressurised.

21. A floating roof according to claim 19 inwhich the central frameworkcomprises a central sleeve equipped with a stabiliser, the frameworkbeing adapted to mate with an external mast.

22. A floating roof according to claim 21 in which the stabilisercomprises a horizontal plate.

23. A floating roof according to claim 1 in which the floating rimcomprises a plurality of linear elements connected togethersubstantially in the form of a polygon.

24. A floating roof according to claim 23 in which the linear elementsare connected to one another by means of sleeves on the exterior part ofwhich are plates,

adapted so as to connect with the tensioning cables.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. 3,885,699 DATED May 27, 1975 |NVENTOR(S) Nestor Leon Pladysltis certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

In Column 7, Claim line 45, delete "level" and insert -1ever--.

Signed and Scaled this Twelfth Day of April 1977 [SEAL] Attest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oflatentsand Trademarks

1. A floating roof suitable for use with a large capacity storage tankcomprises (i) a central framework and at least one floating rim, (ii)tensioning cables connected from said central framework to said floatingrim, (iii) a covering sheet adapted to be supported by said centralframework and said floating rim, and (iv) a seal comprising anarticulated collar pivotally attached to the outermost floating rim,said articulated collar being fitted with a first inflatable attachmentand a second inflatable attaChment which is nearer to the pivot than thefirst inflatable attachment, said attachments being on opposite sides ofsaid articulated collar and being adapted so that upon inflation of saidinflatable attachments, the second inflatable attachment is pressedagainst the rim and by a level action presses the first inflatableattachment against a surrounding wall.
 2. A floating roof according toclaim 1 wherein there are a plurality of rims and all the rims arelocated outside the said framework.
 3. A floating roof according toclaim 1 in which at least one of said rims is located within the area ofsaid framework.
 4. A floating roof according to claim 1 in which eachtensioning cable passes over a pulley adapted to fit on the rim.
 5. Afloating roof according to claim 1 in which the covering sheet slopesdown towards the central framework.
 6. A floating roof according toclaim 1 in which the said sheet is fabricated from a plastics material.7. A floating roof according to claim 1 in which the floating rim issubstantially circular in form.
 8. A floating roof according to claim 1in which the rim is substantially circular in cross section.
 9. Afloating roof according to claim 1 in which the tensioning cables arearranged so that the rotational forces applied to adjacent elements ofthe rim are in opposition.
 10. A floating roof according to claim 1 inwhich the covering sheet is adapted to permit collection and removal ofa liquid which may fall on its surface.
 11. A floating roof according toclaim 1 in which the central framework at least one vertical float. 12.A floating roof according to claim 11 in which said central frameworkincludes a plurality of vertical floats; each upper part of said floatsis connected together and each lower part of said floats is connectedtogether by means of upper and lower crowns, respectively.
 13. Afloating roof according to claim 12, in which each tensioning cable isattached to the upper and lower crowns of the central framework.
 14. Afloating roof according to claim 1 in which the covering sheet comprisespanels of shaped material that are juxtaposed and interconnected.
 15. Afloating roof according to claim 14 in which the panels of shapedmaterial are connected together, by means of junction pieces of suitableshape, along the edges of said panels.
 16. A floating roof according toclaim 14 in which the panels of material have corrugations substantiallyat right angles to the elements of the rim.
 17. A floating roofaccording to claim 1 in which the floating rims are fitted withstabilizers for supporting the roof on the bottom of the tank when thetank is empty.
 18. A floating roof according to claim 17 in which thestabilisers comprise substantially vertical legs fitted with horizontalplates.
 19. A floating roof according to claim 1 which comprises onefloating rim, said rim being connected to the central framework bytensioning cables attached to an upper and a lower part of saidframework and a covering sheet formed in two parts, one part of which issupported by the upper section of said tensioning cable and the otherpart being attached to the central framework and floating rim, the twoparts of said sheet being adapted to combine with the rim to form aleakproof enclosure.
 20. A floating roof according to claim 19 in whichthe leak-proof enclosure is adapted to be pressurised.
 21. A floatingroof according to claim 19 in which the central framework comprises acentral sleeve equipped with a stabiliser, the framework being adaptedto mate with an external mast.
 22. A floating roof according to claim 21in which the stabiliser comprises a horizontal plate.
 23. A floatingroof according to claim 1 in which the floating rim comprises aplurality of linear elements connected together substantially in theform of a polygon.
 24. A floating roof according to claim 23 in whichthe linear elements are connected to one another by means of sLeeves onthe exterior part of which are plates adapted so as to connect with thetensioning cables.